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antedeluvian
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Ac coupling
antedeluvian   6/13/2014 11:36:07 AM
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Max

If you want to get rid of the DC component of the signal just use a capacitor in series with the signal. On t'other side you need a largish resistor to ground if you are looking to ground reference it, but you can bias it to whatever DC value you like by using a resistor divider.

Remember to choose the value of the capacitor for a low impedance and that it will interrelate with the aformentioned resistors. Today you can get ceramic caps in the 10uF range or more, but in the old days you had to use polarized ones leading you to try and figure out which side should be positive, Of course you could depolarize by placing two in series in opposing polarity.

I am surprised there wasn't a DC bias. From my understanding (which is very little) of a TI seminar on the audio amplifiers in phones etc. the voltage swing is doubled by driving the output in opposite phases across the speaker, much like the driver on RS485 and as I recall it ran on a single supply. Obviously I misunderstood.

 

 

Max The Magnificent
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Re: Ac coupling
Max The Magnificent   6/13/2014 1:20:52 PM
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@Aubrey: If you want to get rid of the DC component of the signal just use a capacitor in series with the signal.

Absolutally -- in fact the circuit I'm using with the MSGEQ7 chips from SparkFun has just such a decoupling capacitor (I'll be discussing thsi circuit in a future column) -- the main thing at this point is that I wanted to use my frequency generator to replicate the signal coming out of the iPad -- if there had been a DC offset I woudl have replicated it, even though my series capacitor would have (effectively) taken it out again.

Max The Magnificent
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Re: Ac coupling
Max The Magnificent   6/13/2014 1:22:04 PM
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@antedeluvian: I am surprised there wasn't a DC bias...

Sonce I know so little about audio, I wouldn't have been surprised either way :-)

elizabethsimon
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Re: Ac coupling
elizabethsimon   6/13/2014 5:07:57 PM
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The good people at Apple, knowing that folk will merrily plug the headphone jack into all kinds of speakers probably already included the capacitor in series to get rid of any DC offset. At least that's what I'd do if I was in charge....

zeeglen
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Re: Ac coupling
zeeglen   6/13/2014 5:17:31 PM
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A DC bias would offset the diaphragm of the headphones and reduce its mechanical travel limits, ie mechanical clipping of the positive peaks.  Not good.

Max The Magnificent
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Re: Ac coupling
Max The Magnificent   6/13/2014 5:20:42 PM
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@zeeglen: A DC bias would offset the diaphragm of the headphones and reduce its mechanical travel limits...

Unless you have a pair of the old Mark 4s with the counter-ballanced notchet-tattles.

David Ashton
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DC offsets....
David Ashton   6/13/2014 7:27:12 PM
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Anyone who claims to be offering Audio and delivers it with a DC bias needs to be shot (or at least have a swiftly administered kick to the rear end).     As Antedeluvian says, if you do have one you can use a capacitor to get rid of it.  While Antedeuvian's suggestion of a ceramic cap is good, you could also use an electrolytic if you know which way it's biased (or you can get non-polarised electrolytics these days).  though some purists do frown upon electrolytics for Audio, I've used them for years and had no real problems unless they go dry...which takes many years....

Most audio equipment has the standard 3.5 mm stereo jack output which implies a common ground.  If this is the case then there cannot be a bridge output circuit, in which both speaker terminals are live and have a DC bias (if you had, say, a 10V supply, then both terminals would be at 5v in the absence of a signal.  When you have a signal the terminals would move in opposite directions).

Bridge amps are commonly used in things like phones where you only have one speaker and you don't have to worry about common grounds.  And in high-power audio amps - if you double the effective output swing with a bridge, you quadruple the power!  And you can usually use seperate wires for each speaker - again you don't have to worry about common grounds.

I think most phones these days would use class D amps - basically using pulse width modulation - which has far higher efficiency.  I found a couple of class D amps in a TV I pulled apart recently.

betajet
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Re: DC offsets....
betajet   6/13/2014 8:27:25 PM
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David says: Anyone who claims to be offering Audio and delivers it with a DC bias needs to be shot (or at least have a swiftly administered kick to the rear end).

I say they deserve a good thumping.  After all, that's what they're doing to the speakers :-)

rich.pell
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Impedances
rich.pell   6/13/2014 10:09:31 PM
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"I also know that you are really not supposed to use a headphone output to drive an amplifier directly, because they tend to have different impedance characteristics."

Impedance differences are not the issue.  A headphone output can certainly be used for driving an amplifier input, but - unlike a fixed-output-level line-level output - simply needs its output level to be set properly as headphone output capabilities will vary.  In the case of an iPod/iPhone headphone out, there may not be much difference between its max voltage output and that of a standard line-level output.

sa_penguin
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C Moy
sa_penguin   6/15/2014 12:23:37 AM
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The most common headphone amp circuit is called the "C Moy". When you have a signal with zero DC bias you get the fun of making virtual earths and +/- power supplies. You also get more problems, when you put it in a car - and try running everything off a single 12V battery.

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